Magnetic amplifier



Dec. 11, 1951 E. G. DOWNIE MAGNETIC AMPLIFIER Filed Feb. 28, 1950 Invent or Emerson G. Dvvnie His Attorney Patented-Dec. 11, 1951 2,578,405 I MAGNETIC AMPLIFIER Emerson G. Downie, Fort Wayne, Ind., assignor to General Electric New York Company, a corporation of Application February 28, 1950, Serial No. 146,847

' 2 Claims. (01. 323-429) This invention relates to and more particularly netic amplifiers which rectifiers.

An object of the invention is to provide a new and improved magnetic amplifier.

Another object of this invention is to provide a new and improved magnetic amplifier whose output is compensated for supply voltage compensation.

A further object or the invention is to provide a new and improved compensated cascaded magnetic amplifier and dry contact rectifier control system.

The invention will be better understood from the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, which is 9. diagrammatic illustration oi. a preferred embodiment of the invention, the principal parts of the device comprise a main control thermostat i, of the variable impedance type, connected to control a first stage magnetic amplifier 2 which in turn is connected to control a second stage magnetic amplifier 3 whose output circuit 4 constitutes the output portion of the illustrated device or system. The power supply for the system is indicated generally at 5 and comprises a power supply transformer 6, an alternating current power circuit l-8 connected to the secondary winding of the transformer G and a lull wave dry contact bridge connected rectifier 9 for sul plying current to a direct current supply circuit iii-l l.

The illustrated system is suitable for general purpose amplifier applications and is particularly adapted to automatic temperature control of the modulation type such, for example, as in house heating where a room thermostat, through suitable amplifiers, controls the position 01' a valve controlling the amount or gas fuel supplied to a gas furnace. Such modulation control is to be distinguished from the conventional on-ofl control of ordinary oil burner type domestic heating systems and automatically controls or modulates the flow of fuel in accordance with the need for heat at all of fuel flow is proportional to the deviation of the room temperature from the thermostat setting.

The thermostat I comprises a bimetal member l2 attached to a magnetic core member ii for a pair of high reactance coils I4 and I5.

magnetic amplifiers to improvements in maginclude dry contact type Changesin room temperature cause deflection times so that the rate 2 oi the bimetal strip l2 thus moving the core I! relative to the coils l4 and I5 so that the reluctance of these coils will be varied inversely. A suitable stop is may be provided for limiting the movement of the bimetal strip l2 in one direction beyond a position in which the reactance oi the coils I4 and I5 is equal.

Connections between thermostat i and the first stage magnetic amplifier 2 are made by means oi. a terminal board ll.

The, first stage magnetic amplifier 2 comprises a saturable three legged magnetic core ll, the outer legs of which carry variable reactance windings l2 and 20 and the center leg of which carries a control winding 2| and a bias winding 22. The control winding 2| is connected to the thermostat i by means oi a full wave dry con tact rectifier 23 of any suitable type, such as the copper oxide type or the selenium type, and a bridge circuit, two of whose arms are the windings l4 and i5, 01 the thermostat and whose other two arms are resistors 24 and 25 and parts of an adjustable resistor 28. Input terminals or the bridge are connected by conductors 21 and 28 to the main alternating current supply circuit '!8 and output terminals of the bridge are connected by conductors 29 and 30 to input terminals of the rectifier 28 minals are connected to the control winding 2|.

For providing compensation oi the amplifier 2 for variations in supply voltage and also for providing a more refined or complete compensation of the entire system for variations in supply voltage in a manner which will be described below in connection with the operation of the system, the bias winding 22 is energized from the main alternating current supply circuit 1-8 through a resistive potential divider "-32 and a full wave dry contact rectifier 33 of the same type and rating as the rectifier 23.

The second stage magnetic amplifier 2 is generally similar to amplifier 2 and comprises a three legged core 34 having variable reactance windings 35 and 36 on its respective outer legs together with a control winding 31 and a bias winding 38 on its center leg. The control winding 31 is energized in accordance with the output of the first stage amplifier 2 by means of a full wave rectifier 39 across whose output terminals winding 31 is connected. The rectifier 39 is in turn controlled by the variable reactance windings I9 and 20 of the first stage amplifier and the current supp y for these two windings is obtained by connections 40 and 4| to the main alternating current suppl circuit 1-8. The

whose output terbias winding 38 of the second stage amplifier is energized through a current limiting resistor 42 from across the output terminals of the rectifier 9.

The variable reactance coils 35 and 36 are connected across the alternatingcurrent supply -'I--8 by way of conductors 43 and 44 by way of a rectifier 45 whose output supplies direct current to the output circuit 4.

The general operation of this system is as follows: Assume that the room temperature is above the setting of the thermostat and this causes the bimetal strip 12 to move against the stop l8 so as to position the core I3 symmetrically with respect to the coils I4 and I5 so that they have equal reactance. Under these conditions, it will further be assumed that the bridge circuit including the windings I4 and I5 is balanced so that the output of the bridge is zero and, consequently, no current flows in the control winding 2|. The magnetizing efiect of the current in the bias winding 22 will under these conditions be unopposed so that the core I8 will be saturated and the output of the amplifier 2 will be a maximum. Consequently, the current in the control winding 31 of the second stage amplifier 3 will be a maximum. The bias coil 38 and the current limiting resistor 42 are so selected that the magnetizing effect of the bias coil 38 under these conditions will neutralize the magnetizing effect of the control winding, 3'! with the result that the core 34 will be unsaturated so that the output of the second stage amplifier 3 will be a minimum. Consequently the current in the output circuit 4 will be a minimum. Therefore, a gas fuel control valve (not shown) connected to the circuit 4 will be closed under these conditions, thus, in effect, cutting off the supply of heat to the thermostat I.

If now the temperature at the thermostat I falls slightly, the resulting deflection of the bimetal strip l2 to the left will move the core l3 so as to unbalance the bridge circuit containing the windings l4 and I5 with the result that current fiows in the control coil 2|, which current, in general, will be proportional in magnitude to the drop in the temperature from the setting of the thermostat. This current will cause the control winding 2| to produce a magnetizing eilect opposing that of the bias winding 22 so that there will be a desaturating efl'ect on the first stage amplifier 2 thus decreasing its output. The resulting decrease in current in the control winding 31 of the second stage amplifier will thus have a saturating eilect on the second stage amplifier because the magnetizingeffect of the bias coil 38 will now predominate over the magnetization produced by the control coil 31 so that the output or the second stage amplifier increases with the result that the valve (not shown) energized in proportion to the current in the output circuit 4 will be opened in proportion to the increase in output current thus eventually causing an increase in temperature at of inherent compensation of the system for unavoidable fluctuations in supply voltage delivered by the transformer 6. Thus, for example, if the supply voltage increases, there tends to be a direct increase in current in the output circuit 4 by reason or the fact that the voltage applied to the windings 35 and 35 by way of the supply circuit 'I-3 increases. However, at the same time this same voltage increase is also applied to the variable reactance windings l9 and 20 of the first stage amplifier with the result that the output of the first stage amplii.er tends to increase directly in response to an increase in supply voltage, and this increase in the output of the first stage amplifier produces a counteracting decrease in the output of the second stage amplifier. However, this inherent compensation due to the reverse action of the two stages has been found not to be adequate in many cases for a number of reasons. The most important of these is due to the fact that it is necessary that the coils l4 and i5 of the thermostat carry a minimum amount of current so as to reduce the electromagnetic effects on the core I3 as much as possible and also so as to reduce objectionable noise or hum produced by such current, with the result that the rectifier 23 operates on the non-linear portion of its characteristic where the forward re sistance of its elements varies appreciably with variations in current therethrough. Consequent ly, variations in supply voltage will, in effect, be magnified by the non-linearity of the rectifier 23 so that the output of the system from the circuit 4 is quite sensitive to variations in supply voltage. Furthermore, variations in supply voltage also produce a direct effect on the current in the bias winding 38 and this effect tends to supplement the direct eilect on the main variable reactance coils 35 and 36 with variations in supply voltage.

In order to compensate the system for these latter effects, the rectifier 33 matches in characteristics the rectifier 23 and the potential divider 3I32 is so proportioned that the current in the rectifier 33 will be equal to the current in the rectifier 23 during normal or average operating conditions of the system. Consequently, any non-linear response of the control winding 2| to varitions in supply voltage will be neutralized by the corresponding non-linear response of the bias winding 22 to the same voltage fluctuations. In this manner, the system is rendered substantially independent of fluctuations in supply voltage.

Tests show that if the bias winding 22 is energized in parallel with the bias winding 38 from the rectifier 9, then variations in line voltage from minus 15% to plus 10% of a nominal volts will cause a shift in the controlled temperature with a fixed thermostat setting of 4.5 to 5.0 degrees F. However, when the bias winding 22 is energized in the manner shown, by means of the rectifier 33, potentiometer 3l32 combination so as to match the non-linear characteristics of the energization of the control winding 2|, then under the same variations in operating conditions the temperature shift is less than 2 degrees F.

This arrangement also compensates the system for aging efiects in rectifier 23 because such efggcts will be matched in corresponding rectifier While there has been shown and described a particular embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A compensated magnetic amplifier comprising, in combination, a saturable magnetic core, a controlled winding on said core for connection in an output power supply circuit having alternating current power supply terminals, output terminals and a rectifier; a main control winding on said amplifier; an input circuit for said amplifier comprising a main control variable impedonce, a rectifier and said main control winding, said input circuit being connected across said power supply terminals; a bias control winding on said amplifier; and means including a serially connected rectifier and an impedance for ener- :izing said bias control winding from said power supply terminals, said rectifier for energizing said bias control winding being a dry contact rectifier, the impedance associated with said last mentioned rectifier being of such value that said rectifier operates over a non-linear portion of its characteristic so as to compensate said amplifier for fluctuations in voltage between said power supply terminals.

2. A compensated magnetic amplifier control system comprising, in combination, power supply terminals for connection to a source of alternating voltage having a nominal value which is sub- Ject to random uncontrolled fluctuations; first and second stage cascaded magnetic devices each having a saturable magnetic core provided with a controlled winding, a main control winding, and a bias control winding; means including a second stage output rectifier for connecting the controlled winding of said second stage device across said power supply terminals in series with, a pair of output terminals for said amplifier; means including a supply rectifier for connecting the bias control winding of the second stage device across said power supply terminals; means including a first stage output rectifier for connecting the controlled winding of the first stage device across said power supply terminals in series with the main control winding of the second stage device; a bridge circuit including a pair of variable reactance main control arms; means for connecting said power supply terminals respectively to input terminals of said bridge circuit; an input rectifier for connecting the main control winding of said first stage device to output terminals of said bridge; a compensating dry type rectifier having a non-linear impedance characteristic at low currents; and means including an impeder for con necting said compensating rectifier in series with the bias control winding of the first stage device across said power supply terminals for compensating the output of said amplifier for said fluctuations in said source of alternating voltage.

EMERSON G. DOWNIE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

